Power steering systems for passenger and industrial vehicles often use an hydraulic system to provide power boost to the steering mechanism to assist the driver in steering. Hydraulic powered steering systems require a fluid pump in order to provide pressure for the fluid in the system. The pump requires a relatively large supply of electrical energy. In electric vehicles, an hydraulic pump drains the stored electrical energy from the vehicle's battery reducing the effective range of the vehicle. Even in gasoline powered vehicles, where electric power is more readily available, the power requirments of the pump reduce the gas mileage of the vehicle.
In addition, these steering systems generally have a valve which directs fluid power from a pump to a steering actuator when the steering wheel is rotated to steer the vehicle left or right. In general, there is a small range, typically 1.degree. to 2.degree. where the steering wheel has free play because the valve does not respond immediately to the motion of the steering wheel; it is desirable to minimize this play. Beyond this play is an active region of 4.degree.-6.degree. where boost is proportional to deflection.
In an effort to eliminate the drawbacks of hydraulic systems, some manufacturers have turned to electrical power assisted steering systems. Electrical systems are more efficient, requiring less energy than the power steering pump of the hydraulic systems. The system is typically greater and offers flexibility in location of components. A typical electrical power steering system uses an electric motor coupled between the positive and negative terminals of the automobile battery. A control circuit controls the operation of a set of switching transistors, usually four, energizing one pair of the transistors to turn the motor in a first direction, and energizing the second pair to turn the motor in the opposite direction.
While the electrical power steering systems have advantages over the hydraulic systems, the prior art systems also have certain drawbacks. For example, relatively complex control circuitry is required to control the operation of the switching transistors because two transistors must be energized to turn the motor in a given direction.